Now that the Committee on Foreign Investment has given its approval, only a vote among MetroPCS shareholders to sign off on the deal is holding up the merger, which is set to be voted upon on April 12.

In a letter, the Justice Department's National Security Division asked for a deferring of the merger as part of a common procedure when a non-U.S. company is involved in a takeover of or merger with a U.S. firm, as is the case here. (A similar letter was sent to the FCC regarding the Softbank-Sprint deal.)

The T-Mobile-MetroPCS deal will go ahead in a complicated stock-and-cash deal, in which Deutsche Telekom, T-Mobile USA's Germany-based parent company, will own 74 percent of MetroPCS' common stock. The move initially came as a bit of a surprise considering rumors suggesting that the German company was trying to leave the U.S. market.

According to earlier reports, T-Mobile USA plans to cut a large number of staff at its Bellevue, Washington headquarters once the merger completes

That comes after around 4,200 jobs were cut across the cellular network during the last calendar year.

The fully merged U.S. carrier, which would have a combined subscriber base of roughly 42 million users, had an enterprise value of about $30.5 billion, according to Bloomberg, based on MetroPCS' closing price yesterday.